Interaction of tubulin with chromatin proteins. H1 and core histones.

Purified histones, H1 or core histones, induce the aggregation of tubulin. The aggregation process, studied by light scattering at 350 nm and polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate, is dependent on the respective amounts of histones and tubulin; a maximum is obtained at a stoichiometry of 1 molecule of H1 or 2 molecules of core histones/tubulin dimer. At these molar ratios, all tubulin and histone molecules are found in the insoluble material which sedimented at 2000 X g. Increasing H1 or core histones, there is a progressive decrease of light scattering and a concomitant formation of soluble complexes. The minimum soluble complexes between tubulin and H1 or between tubulin and core histones have the same apparent molecular weight of 150,000-160,000; these complexes consist of one tubulin and two H1 molecules or one tubulin and four core histones. The tubulin-histone interaction is an almost instantaneous reaction which can, however, be slowed down by increasing the ionic strength of the medium. NaCl (0.5 M) decreased by 50% the formation of tubulin-H1 insoluble complexes but slightly affects the core histones-tubulin complex formation. Histones can be classified by their ability to form insoluble complexes with tubulin: H4 = H3 greater than H2B = H2A greater than H1. The reactivity of histones seems to be related to their lysine/arginine content. Electron microscopy revealed that insoluble polymers resulting from the interaction of tubulin with H1 or core histones are similar and consist of unordered aggregates of 35-40 nm ring structures.